1. Field of the Invention
This invention is directed to a focusing and tracking detection scheme for use in an optical data storage system.
2. Description of the Prior Art
In a conventional optical data storage system, a beam of radiation is focused onto an information carrying surface by an objective lens. In one embodiment the surface is composed of circular grooves or one continuous spiral on an optical disk. The information carried in the grooves or spiral modulates the beam of radiation and reflects it onto a photodetector. The photodetector produces an electrical signal based upon the amount of radiation impinged thereon. This electrical signal corresponds to the information contained on the optical disk. The electrical signal is then further processed and the information is eventually reproduced in the desired form (i.e. audio, visual, electrical).
In order for the information stored on the optical disk to be reproduced accurately, it is important that the objective lens accurately focus the beam of radiation on the optical disk and that the focused beam of radiation accurately track the circular or spiral grooves. For example, if the information carrying surface moves away from the objective lens, the beam of radiation will be converged at a point slightly in front of the information carrying surface and be out of focus. Conversely, if the information bearing surface moves closer to the objective lens, the beam of radiation will be converged at a point slightly behind the information carrying surface and also be out of focus. Additionally, if the circular or spiral grooves on the information carrying surface move radially with respect to the beam of radiation, a tracking error will result and the beam may not be properly modulated. Therefore, both tracking and focus errors must be detected and the objective lens must be positioned to correct such errors.
Focus and tracking errors have been reduced by placing the objective lens on a servomechanism which is movable with respect to the information carrying surface. The servomechanism is controlled by focus error and tracking error devices. These devices are made up of several photodetectors which detect radiation reflected from the information carrying surface. The amount of radiation on each photodetector is converted to an electrical signal which is combined with electrical signals from the other photodetectors. Combinations of these various signals provide focus error and tracking error signals which control the servomechanism. Typically, the focus and tracking error devices consist of a quadriture detector of the type disclosed in the Stewart U.S. Pat. No. 4,065,786.
Prior systems have sometimes used several different detector devices, one to detect focus errors and the other to detect tracking errors. For example, the Elliott et al U.S. Pat. No 4,152,586 discloses a concentric detector. However, the detector in Elliott et al is only capable of detecting focus errors and an additional detector is needed to detect tracking errors.
In prior inventions where there has been a combination of these detectors, the focus error signal and tracking error signal have been detected from the same radiation beam. In all the instances utilizing this type of detector, however, some additional special optical element has always been needed for redirecting or blocking a portion of the radiation beam. This brings a complexity which makes alignment of the system more difficult and in some instances, for instance in astigmatic focusing, results in high crosstalk between tracking and focus error signals.